1. Field of the Invention
The present invention relates to a zoom optical system, and an imaging apparatus provided with such a zoom optical system.
2. Description of the Related Art
In recent years, mobile electronic devices, such as digital cameras (still-video cameras), mobile phones and smart devices (smart phones or tablet computers, etc.) which are equipped with a camera, have become widespread. There has been a desire to slim down the imaging apparatus, in which a camera is provided, in a depthwise direction with respect to object-emanating light rays that are incident thereon, and to miniaturize the imaging apparatus, in order to improve portability and freedom of design.
Accordingly, various types of zoom optical system have been proposed, in which a reflective optical member is provided for suppressing the “thickness” of the zoom optical system (i.e., the length of zoom optical system with respect to the forward/rearward direction of the mobile electronic device) in a depthwise direction with respect to object-emanating light rays that are incident thereon. For example, in each of Patent Literature Nos. 1 through 7, a zoom optical system provided with four lens groups (namely, a negative first lens group, a positive second lens group, a positive third lens group and a positive fourth lens group, in that order from the object side) is disclosed in which a slimming down in a depthwise direction thereof is achieved by providing a prism which is configured to bend the optical path.
Patent Literature No. 1: Japanese Unexamined Patent Publication No. 2012-163855
Patent Literature No. 2: Japanese Unexamined Patent Publication No. 2010-44395
Patent Literature No. 3: Japanese Unexamined Patent Publication No. 2008-8978
Patent Literature No. 4: Japanese Unexamined Patent Publication No. 2008-96661
Patent Literature No. 5: Japanese Unexamined Patent Publication No. 2006-251529
Patent Literature No. 6: Japanese Unexamined Patent Publication No. 2004-4533
Patent Literature No. 7: Japanese Unexamined Patent Publication No. 2010-49263
However, in each of the zoom optical systems of Patent Literature Nos. 1 through 7, there is a problem with the overall length of the zoom optical system being too long. For example, in each of the zoom optical systems of Patent Literature Nos. 1 through 7, a value equal to the overall length of the optical system divided by the maximum image height, at the long focal length extremity, is in the range of 12.4 through 19.0, which is a large value.
Furthermore, in each of the zoom optical systems of Patent Literature Nos. 1 through 7, the length of the prism along the optical axis is long, thereby causing the body (in which the zoom optical system is housed) to become thick in the depthwise direction (forward/rearward direction) thereof. For example, in each of the zoom optical systems of Patent Literature Nos. 1 through 7, a value equal to the length of the prism along the optical axis divided by the maximum image height, at the long focal length extremity, is in the range of 1.9 through 4.0, which is a large value.